The present invention relates to optical storage disks, and, in particular, to methods and apparatus for the retrieval of high density information in optical storage disks with high readout bit-rate.
The density of optical memory is normally described in two dimensional formats and is typically quantified in bits per square millimeter. The upper limits on information density are set by the diffraction of light. Specifically, the minimum diameter of a light spot formed at a focal point is about .lambda./(2NA), where .lambda. is the wavelength and NA is the numerical aperture of the focused beam. Consequently, the information density is approximately (NA/.lambda.).sup.2 corresponding to 10.sup.9 to 10.sup.10 bits on one surface of a typical 4.7 inch (120 mm) diameter optical disk. Optical disks are available on which information may be stored on both sides of the disk. The description herein is equally applicable for both single-sided and double-sided optical disks.
Advances in computer technology call for increased memory capacity and shortened access time. Current work in the optical storage field is geared towards quadrupling the capacity of the disks by utilizing blue-green diode lasers. Significant improvement in memory capacity requires increasing the volume density of the storage medium. Presently, such storage density increases are achieved by exploiting optical tapes. However, optical tapes are serial access devices with limited access speed which greatly reduces the attractiveness of this approach relative to optical disks whose geometry makes it possible to use random access techniques.
The optical disk, as a two-dimensional optical storage device, is currently the most widespread physical format for optical storage. The volume data density of optical disks can be increased by adding a third physical dimension. This can be done by using a multilayer optical disk, i.e., by axially stacking a number optical disks. In order to utilize the third physical dimension, an unconventional optical head must be used.
Several stacked optical disk systems have been proposed. IBM Almaden Research Center of San-Jose, Calif., propose a unique volumetric method for increasing optical disk capacities which is disclosed in U.S. Pat. Nos. 5,202,875 and 5,255,262, incorporated herein be reference for all purposes as if fully set forth herein. The IBM approach involves bonding together individual disks in a stack with spacers provided between adjacent disks to define a gap between adjacent disks. At any one time, a movable lens in the optical disk drive focuses a laser on one surface of one of the disks in order to read data. The focus of the laser is changed repeatedly to sequentially read data from the various disk surfaces. Each disk, or at least all but the farthest disk from the laser source, must be partially transparent so that the laser can be used to read a disk which lies beyond one or more other disks. Each disk surface, however, must also be sufficiently reflective to allow the data to be read.
Unfortunately, there are several limitations in using the IBM approach. By increasing the number of disks, the signal to noise ratio (SNR) is generally reduced and the interlayer crosstalk becomes significant. Moreover, the time required to focus the optical head on the desired disk surface, which involves accelerating the head from a fixed position, displacing it, and decelerating the head to a new rest position, requires significant time and greatly limits the data access time of the device.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method and apparatus which will alleviate the above limitations by providing multilayer optical disk storage with high volume storage capacity and high bit-rate readout of the stored data.